1. Field of the Invention
This invention relates to an automatic focus adjusting apparatus in a camera or the like, and in particular to a driving method having the function for pursuing, when an object is moving, the moving object so that the moving object is always in the in-focus state.
2. Related Background Art
The construction of a conventional automatic focus detecting apparatus which does not effect pursuit will first be described with reference to FIG. 1 of the accompanying drawings. A light emanating from an object a passes through an imaging optical system (a photo-taking lens) L and is directed via a quick return mirror M onto a viewfinder screen S which is normally in a position conjugate with a film surface F. Also, some light transmitted through the central translucent portion of the quick return mirror M is directed via sub-mirror S.M. to focus detecting means 101. The focus detecting means 101 is of a known construction and is comprised of a focus detecting optical system, a charge storage type image sensor, a focus detection calculating portion and an image sensor drive control portion. After the termination of the charge storage, the defocus amount is intermittently calculated in the focus detection calculating portion. This defocus amount is an amount corresponding to the distance along the optic axis between the surface conjugate with the film which is a predetermined imaging plane and an image surface of the imaging optical system L.
Control means 102 receives the data regarding the defocus amount from the focus detecting means 101, drives the motor of lens drive means 104 to move a focus adjusting optical system included in the imaging optical system L and controls it so that the predetermined imaging plane and the image surface of the imaging optical system become coincident with each other. Where the motor of the lens drive means is not of the type in which accurate control of the drive amount is possible by an input signal, the control means 102 controls the driving of the focus adjusting optical system by the utilization of the feedback pulse of monitor means 103 comprised of a photointerrupter or the like and detecting the amount of movement of the focus adjusting optical system. Of course, where like a pulse motor, accurate control of the drive amount is possible by an input signal, the monitor means can be replaced by means for detecting an input pulse, or means equivalent thereto.
Generally, it is rarely practised to move the entire imaging optical system to thereby accomplish automatic focus adjustment, and it is the usual practice to move a focus adjusting optical system which is a part of the imaging optical system to thereby accomplish focus adjustment. In this case, the amount of movement of the focus adjusting optical system does not agree with the amount of movement of the image surface of the imaging optical system. Accordingly, actually, the value regarding the ratio between the feedback pulse number output by the monitor means 103 and the amount of movement of the image surface is pre-recorded in lens information generating means 105, and the control means 102 reads the value of this ratio from the lens information generating means 105 and calculates the necessary pulse number corresponding to the necessary amount of movement of the image surface (the defocus amount), and effects driving until the feedback pulse equals this necessary pulse number.
However, this point has no relation with the essence of the present invention, and in the ensuing description, the imaging optical system will be represented by an imaginary single lens so that the essential point of the present invention can be easily understood, and the description will be given on the assumption that the amount of movement of the imaginary single lens and the amount of resulting movement of the image surface are equal to each other.
Of course, even in a single lens or a lens in which all lens groups are axially movable, the above assumption is not appropriate at all if the distance to the object is very close and accordingly, in a macro-lens or the like, it becomes necessary to change the value of said ratio stepwise in conformity with the amount of axial movement of the lens, and the lens information generating means 105 needs to be capable of performing this. For simplicity herein what is concerned with the value of said ratio is left to the lens information generating means, and the description will be given using an imaginary single lens so that the description will be readily understood.
FIG. 2 of the accompanying drawings fixes the coordinates in such an imaginary single lens L for the convenience of description, and shows the locus of the imaging plane of the moving object in that case (solid line P) and the locus of the predetermined imaging plane conjugate with said film surface (dotted line Q). In FIG. 2, the abscissa represents time, and the ordinate represents the distance along the optic axis between said imaging plane and said imaginary single lens. In the figure, the coordinates t.sub.n, x.sub.n represent the time t.sub.n when the charge storage of the focus detecting means 101 is started and the then position x.sub.n of said predetermined imaging plane, the coordinates t.sub.n ', x.sub.n ' represent the time t.sub.n ' when the storage is terminated and the then position x.sub.n ' of said predetermined imaging plane, and the coordinates t.sub.n.sup.0, x.sub.n.sup.0 represent the time t.sub.n.sup.0 when the focus detection calculation is terminated and the then position x.sub.n.sup.0 of said predetermined imaging plane.
FIG. 2 shows the manner of the focus adjusting operation in the so-called intermittent drive, and more specifically shows the manner in which the image surface is moving as indicated at a.sub.1, a.sub.2 and a.sub.3 while pursuing the object as it moves as indicated at a.sub.1 ', a.sub.2 ' and a.sub.3 ' with time.
When as a result of the first calculation, the value D.sub.1 regarding the difference in distance between the imaging plane a.sub.1 of the object and the film surface b.sub.1 is output from the focus detecting means 101 at time t.sub.1.sup.0 ', the control means 102 drivingly controls the lens L to offset this defocus amount D.sub.1 in the manner described previously. However, in the meantime, the object a.sub.1 ' is moving to a.sub.2 ' and therefore, even if the lens is driven by D.sub.1 at time t.sub.2 and the lens driving is stopped, the imaging plane already moved to a.sub.2 by the midpoint of the next storage time, and as a result of the second calculation, the value D.sub.2 regarding the difference in position between a.sub.2 and b.sub.2 is output from the focus detecting means 101 at time t.sub.2.sup.0. Thereupon, the control means 102 effects control to offset this defocus amount D.sub.2 in the manner described previously, but even if the lens driving is stopped at time t.sub.3 and the predetermined imaging plane is brought to a position x.sub.3, the in-focus state will not take place because the object a.sub.2 ' has already moved to a.sub.3 '.
Thereafter, a similar matter as shown in FIG. 2 is repeated. Assuming that the object has not moved and that there have been no errors, it ought to have been possible that the object image surface and the predetermined imaging plane are made coincident with each other in one cycle of storage, calculation and drive, and even if 10% to 20% error has been included in the defocus amount which is the result of the calculation, it ought to have been possible that the object image surface and the predetermined imaging plane are made substantially coincident with each other in two to three cycles.
However, when the object is moving in the direction of the optic axis as shown in FIG. 2, the distance between the object image surface a.sub.n and the predetermined imaging plane b.sub.n gradually becomes smaller in the first two to three cycles, but the distance between the two thereafter is kept at a predetermined value determined by the speed of movement of the object image surface and the responsiveness to the automatic focus adjusting apparatus, and the state of follow-up with the in-focus state remaining unattainable continues.
What has been described above is an example of the case of the so-called intermittent drive in which the charge storage, the focus detection calculation and the lens driving are effected in the named order without overlaping one another. As another driving method, the so-called overlap drive in which the lens driving is effected along with the charge storage and the focus detection calculation is known from U.S. Pat. No. 4,387,975, but it is clear that even in such case, follow-up must be effected for a moving object. Thus, in the conventional driving systems, there is the disadvantage that follow-up is effected in the case of a moving object, and to solve this problem, the applicant has proposed in Japanese Laid-Open Patent Application No. 214325/1985 an automatic focus detecting apparatus having the object pursuing function which presupposes the overlap drive.
The aim of this apparatus is to provide movement detecting means for detecting movement of an object, correct the intermittently calculated defocus amount by the output of the movement detecting means every moment even during the interval period, effect the driving of the lens and pursue the moving object.
This known pursuit system is set up on the premise that the charge storage and the focus detection calculation are always effected alternately with no interval therebetween, and the lens driving is effected during the charge storage and during the calculation along therewith. After the termination of the focus detection calculation, simultaneously with the resumption of the next storage, convergence drive for making said predetermined image surface coincident with the image surface of the object supposed from the calculation is effected, and after the termination of the convergence drive, pursuit driving in which said predetermined image surface is moved in pursuit along the supposed movement of the image surface of the object.
Accordingly, it is often the case that during the storage, change-over takes place from the convergence drive to the pursuit driving and therefore, correction is made of the calculated defocus amount accompanying a variation in the speed of the lens driving during the storage. When the lens is being moved at a predetermined speed during the storage, the correcting process is easy to do because the calculated defocus amount is considered to correspond to the value of the measured distance in the middle of the storage time, but this correction is not easy when the speed of the lens changes during the storage. The aforementioned Japanese Laid-Open Patent Application No. 214325/1985 describes in detail a method of correcting the influence of the change in the speed of the lens with the time when the change in the speed occurs as a parameter.
In this example of the prior art, the case where both the convergence drive and the pursuit driving are at constant speeds is supposed as the premise of the correction, but actually, it is often the case that to satisfy high stopping accuracy and high-speed drive, the lens is controlled so that it is moved at a high speed where the defocus amount is great, and is gradually reduced in its speed to thereby bring it close to the desired position. Accordingly, the convergence drive is not a drive at a predetermined speed, and there is the disadvantage that if it is approximated by a constant speed, the error of the correction becomes great.
As a result, if detection of movement is effected with said indefinite correction added to the intermittently output defocus amount, correct detection of movement cannot be accomplished, and this has led to the disadvantage that the lens driving has a touch of hunting. Even if the pursuit of the object is not effected, if the calculated defocus amount is indefinite, the lens driving has a touch of hunting, but in the pursuit driving, the movement in the future is forecast by extrapolating from the result in the past and thus, the error becomes double by the extrapolation and the tendency toward hunting is increased.